Cross-Talk Coefficient Updating In Vector Transmission

ABSTRACT

Embodiments related to far-end cross-talk coefficient updating in vector transmission systems are depicted and described herein.

BACKGROUND

In many data communication systems data are transmitted by modulatingdigital data onto carriers. Such data transmission systems includesingle-carrier data communication systems such as QAM (Quadratureamplitude modulation) or PSK (Phase shift keying) and multi-carriersystem using a plurality of subcarriers such as OFDM (Orthogonalfrequency division multiplexing) or DMT (Discrete multitone modulation)to transmit data on multiple frequency bands.

Vector data transmission (sometimes referred to as vectoring or vectoreddata transmission) is a technique for coordinated transmission of datafrom a plurality of transmitters to a plurality of receivers via aplurality of transmission links (transmission channels) in order toimprove the transmission. Vector transmission for example reduces theinfluence of cross-talk by providing compensation or precompensation ofthe cross-talk induced onto the signal during transmission over thelink. Vectoring is sometimes also referred to as Spectrum ManagementLevel 3.

One type of cross-talk appearing in such transmission systems is the socalled FEXT (far end cross-talk). For example in Very High Speed DigitalSubscriber Line (VDSL) systems, FEXT can possibly be a major source ofperformance degradation. Typically for such transmission systems, dataare transmitted from a central unit such as a central office (CO) orother provider equipment to a plurality of receivers located indifferent locations, for example in customer premises (CPE), via aplurality of communication channels. FEXT resulting from signals ondifferent channels (lines) transmitted in the same direction results ina reduced data throughput. Through vectoring, signals transmitted overthe plurality of communication channels from the central office orreceived via the plurality of communication channels in the centraloffice may be processed jointly in order to reduce such cross-talk. Inthis respect, the reduction of cross-talk by coordinated transmission ofsignals is sometimes referred to as cross-talk precompensation, whereasthe reduction of cross-talk through joint processing of the receivedsignals is sometimes referred to as cross-talk compensation orcross-talk cancellation. The communication channels which are processedjointly are sometimes referred to as vectored group. Thus, to reduce oreliminate the FEXT influence, FEXT compensation can be used for upstreamcommunication and FEXT precompensation can be used for downstreamcommunication.

For this kind of cross-talk reduction cross-talk coefficients which areparameter describing the cross-talk between the communicationconnections have to be determined and are thereafter stored in order touse these parameters for cross-talk compensation or cross-talkprecompensation. Cross-talk coefficient can be learned for exampleduring an initialization of the system or during a joining of a new lineto the vector group. During operation (showtime) of the system, thecross-talk coefficients are tracked. Learning as well as trackingrequire both a determining of the cross-talk coefficients and anupdating of the previously stored cross-talk coefficients.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 a shows a block diagram according to an embodiment;

FIG. 1 b shows a schematic view according to an embodiment;

FIG. 2 shows a chart diagram according to an embodiment;

FIGS. 3 a to 3 d show sequences according to embodiments;

FIGS. 4 a and 4 b show chart diagrams according to embodiments;

FIG. 5 shows transmission symbols according to embodiments;

FIG. 6 shows a device according to an embodiment; and

FIG. 7 shows a vector transmission device according to an embodiment.

DETAILED DESCRIPTION

The following detailed description explains exemplary embodiments. Thedescription is not to be taken in a limiting sense, but is made only forthe purpose of illustrating the general principles of embodiments whilethe scope of protection is only determined by the appended claims.

In the various figures, identical or similar entities, modules, devicesetc. may have assigned the same reference number.

Referring now to FIG. 1 a, an exemplary embodiment of a vectortransmission system 100 is shown. In the following detailed description,exemplary embodiments are described with respect to a VDSL vectortransmission system. It is to be noted however that the VDSL vectortransmission system is only an exemplary embodiment of a vectortransmission system and that vector transmission system 100 can be ofany other type. Furthermore, it is to be understood that the subscriberlines of the VDSL vector transmission system are only one representationof channels of a vector transmission system and that other embodimentsmay use other communication channels.

The VDSL vector transmission system comprises a DSLAM (DigitalSubscriber Line Access Multiplexer) 102 having a plurality of firsttransceiver units 104 which are coupled to a plurality of channels(lines) 106. Each of the channels of the plurality of channels may forexample be implemented as twisted pair wire. DSLAM 102 may beimplemented in an Optical Network Unit (ONU) such as a Central Office(CO), a cabinet, an exchange or other types of network terminationdevices at the operator's end.

Each of the plurality of channels 106 connects the first transceiverunits 104 at the operator's end with a respective second transceiverunit 108 at a subscriber end. The first transceiver units 104 areimplemented to form with the second transceiver units 108 and channels106 a vector trans-mission system. The second transceiver units 108 mayfor example be integrated in a costumer premise equipment (CPE) such asa home gateway, a router etc. The second transceiver units 108 may belocated at different distances with respect to the transceiver units 104such that the number of channels provided in a cable bundle or cable 110is decreasing with increasing distance from the first transceiver units108 as shown in FIG. 1 a. It is however to be noted that the secondtransceiver units 108 may in other embodiments have a same or nearly asame distance from the first transceiver units.

At the operator's side, a management entity 112 may be provided toprovide management functions such as spectrum management. As will bedescribed later, the management entity 112 may perform also coordinationfunctionality for transmitting symbol sequences for FEXT probing as willbe outlined in more detail below.

While the cable may contain only channels of the vector transmissionsystem, it is to be noted that in some embodiments further lines whichare not part of the vector transmission system, for example ADSL lines,SDSL lines or ISDN lines, which are designated in FIG. 1 a withreference number 106 a may be provided in the cable 110. As shown inFIG. 1 a, the further lines 106 a may terminate at other operator orsubscriber termination locations. For example, the VDSL line may beterminated at a cabinet while the ADSL lines may be terminated at aCentral Office. According to other embodiments of the present invention,all of the channels provided in the cable 110 may be connected to thefirst transceiver units. In such embodiments, all of the channels of thecable may be transmission channels of the vector transmission systemwhile in the embodiment shown in FIG. 1 a, only the channels connectedto the first transceiver units 104 may be transmission channels of thevector transmission system.

Distortion of the data transmission on the vector transmission systemoccurs mainly by two types: distortions which are generated by thevector transmission itself also known as FEXT or self-FEXT anddistortions from outside of the vector transmission system also known asalien noise.

While the alien noise typically can not be compensated, the FEXTdistortions of the vector transmission system can be compensated byhaving knowledge of the signals, i.e. of the data transmitted over thechannels of the vector transmission system.

In upstream direction all of the data send over the channels 106terminate at one of the first transceiver units 104 of DSLAM 102.Therefore, at the receiver side, i.e. at the DSLAM, access to all datatransmitted over the channels 106 can be provided and compensation canbe provided in upstream direction utilizing this data information.

In downstream direction, the data send over the channels 106 arereceived at the respective second transceiver units 108 which aretypically placed at different locations. Typically, no channel betweenthe receiving devices at the different subscriber locations isavailable. In this case, compensation at the subscriber's end can not beprovided as the transceiver unit 108 at one subscriber has noinformation of the data send to the transceiver unit 108 at anothersubscriber.

Reduction or elimination of the FEXT can be achieved in this case byusing a technique known as precompensation. In precompensation, which isalso known as precoding or precancellation, the effect of cross-talkexperienced by a signal during transmission is computed or estimatedprior to transmitting the signal and the signal is modified based onthis information for example by subtracting the calculated cross-talkfrom the transmission signal or adding the negation of the calculatedcross-talk. Then, during the transmission, the transmission signal isexposed to the cross-talk, i.e. the cross-talk adds to the transmissionsignal resulting in the receiving of the original or nearly original,i.e. unmodified or nearly unmodified signal as provided at thetransmitting side except of some other noise added during thetransmission.

For both cross-talk compensation techniques, crosscoupling coefficientsindicating the strength of cross-talk between each resepective line isrequired.

In VDSL, data are transmitted using a multicarrier transmission known asDMT (discrete multitone transmission). Each of the frequency bandsprovided for data transmission is divided into a plurality ofnon-overlapping subcarriers (tones). For each subcarrier, data bits tobe transmitted are represented by a complex number selected of aplurality of predefined complex numbers in a constellation space. Thecomplex number is sometimes referred to as a constellation vector, aconstellation point or a subcarrier symbol. For example, if a 4-QAM(Quadrature Amplitude Modulation) is used for subcarrier k, the complexnumber is selected from the predefined set of {1+j, 1−j, −1+j, −1−j}where j is the imaginary unit. The complex number of each subcarrier isthen transferred to a inverse Fourier transformation unit where a timedomain representation is generated by an inverse Fourier transformationalso known as DMT symbol.

In the above Vector transmission system, FEXT compensation may beprovided independently of the other subcarriers. A model showing thecross-talk for one of the plurality of subcarriers in a vectortransmission system is shown in FIG. 1 b. While FIG. 1 b shows the modelfor one subcarrier, it is to be noted that the model can be applied toeach other subcarrier of a DMT system. It is also to be noted here thatthe above model can also be applied to a system wherein on each channela single carrier modulation is used.

For each subcarrier, the transmission can be described by a MIMO(Multiple In Multiple Out) system wherein the trans-mission system isrepresented by a transmission matrix H. Diagonal coefficients H_(ii) ofthe matrix H which are also known as line coupling coefficients definethe attenuation and distortion due to the line characteristics of linei. Off-diagonal coefficients H_(i,j) represent the FEXT transmissionfunctions and define the FEXT coupling from line i to line j. Asoutlined above, the FEXT coefficients for the respective subcarriers arerequired for FEXT (pre)compensation.

The transmission matrix H(k) for a subcarrier k with L transmissionchannels is mathematically represented according to the above model by

${H(k)} = \begin{bmatrix}{H_{11}(k)} & {H_{12}(k)} & {H_{13}(k)} & \ldots & {H_{1L}(k)} \\{H_{21}(k)} & {H_{22}(k)} & {H_{23}(k)} & \ldots & {H_{2L}(k)} \\{H_{31}(k)} & {H_{32}(k)} & {H_{33}(k)} & \ldots & {H_{3L}(k)} \\\vdots & \vdots & \vdots & \; & \vdots \\{H_{L\; 1}(k)} & {H_{L\; 2}(k)} & {H_{L\; 3}(k)} & \ldots & {H_{LL}(k)}\end{bmatrix}$

As outlined above, in the matrix H(k), the coefficients H_(i,j) with i≠jcorrespond to the FEXT coefficients FEXT_(i,j) while the diagonalcoefficients H_(i,i) correspond to the line coefficients of channel idetermining the transfer function of the transmitted signal on channeli.

In order to provide FEXT compensation, the coefficients of matrix H(k)have to be determined (estimated). This is accomplished according toembodiments of the present invention by transmitting a sequence ofsymbols from the plurality of transceiver units 104 to the plurality oftransceiver units 108 or from the plurality of transceiver units 108 tothe plurality of transceiver units 104 and utilizing only a subgroup ofthe transmitted symbols for providing the updating of the coefficients.As will be described below in more detail, in embodiments the sequenceof symbols is a sequence based on a modulation of a predetermined symbolwith an orthogonal sequence. The sequence may be a sequence of pilotsymbols (SYNC symbols) which are based on a modulation of apredetermined symbol with an orthogonal sequence. Updating of cross-talkcoefficients (cross coupling coefficients) is then performed byutilizing only a selected subset of the sequence of pilot symbols. Aswill be described below in more detail, each of the selected symbolsreceived at a transceiver unit 108 is measured and compared to apredetermined reference to determine a receive error (slicer error). Theslicer error may be error-based such that the slicer error representsthe complex vector indicating in the constellation diagram thedifference between the received symbol and the original transmittedsymbol. In other embodiments, the slicer error may be decision based.Here, the slicer error indicates the sign of the error for the realand/or imaginary part. In some embodiments, a change from an error-basedto a decision-based slicer error may occur during initialization oroperation. The slicer error from the plurality of transceiver units 108is then used to estimate the FEXT coupling coefficients.

As outlined above, the probing (testing) and estimating of thecross-talk for a respective subcarrier is independent to the probing andestimating for other subcarriers. Therefore, the selection of sequencesfor one subcarrier is independent of the selection of sequences for anyother subcarrier. For example, according to embodiments, the samesequences can be used for each subcarrier or different sequences can beused for the respective subcarriers. Furthermore, it is to be noted thatthe above cross-coupling probe signals for one subcarrier may be usedfor a single-carrier system wherein only one carrier is used.

According to one embodiment, the sequence of symbols transmitted are asequence of pilot signals representing synchronization symbols (SYNCsymbols) which are provided in an assemble of data frames also known asa superframe. As shown in FIG. 5, for each channel 1 to L of thesequence of symbols may be a sequence of SYNC symbols OS provided every257th transmitted symbol allowing to transfer 256 data symbolsrepresenting user data in between. SYNC symbols and data symbols may beboth DMT symbols, i.e. a representation of all subcarriers used for DMTmodulation.

According to embodiments, the sequence of SYNC symbols transmitted intime on each channel is generated based on a modulation of predefinedSYNC symbols or SYNC words u₀ with a respective orthogonal sequence.After the end of the sequence modulated by an orthogonal sequence, thesequence is repeated for each channel.

The sequence [x(t1) . . . x(tN)] of SYNC symbols in time transmitted ona channel can be written as the product (multiplication) of apredetermined SYNC symbol u₀ and the orthogonal sequence [s(t1) . . .s(tN)], i.e. [x(t1) . . . x(tN)]=u₀ [s(t1) . . . s(tN)]. It is to benoted that in an embodiment orthogonal sequences modulate the sign ofSYNC symbols. The orthogonal sequences may be periodical and may havethe same length, but they are different by contents and orthogonal toeach other.

Two sequences s′ and s″ are considered pure orthogonal when the dotproduct s′(t1)·s″(t1)+s′(t2)·s″(t2)+s′(t3)·s″(t3)+s′(t4)·s″ (t4) . . .+s′(tN)·s″(tN) of the two sequences (or vectors formed by the sequencess′ and s″) is zero while the dot product of the sequence s′ with itselfand the dot product of the sequence s″ with itself are non-zero.According to one embodiment, the orthogonal sequences are columns orrows of a Hadamard Matrix. A Hadamard matrix is a pure orthogonal matrixwhich contains only +1 and −1 such that any column (or row) isorthogonal to each other column (or row). Columns (or rows) of Hadamardmatrixes are sometimes referred in the art as Walsh-Hadamard sequencesor Walsh sequences.

It is to be noted that in some embodiments orthogonal sequences may alsoinclude what is known as pseudo-orthogonal sequences which are sequenceswherein the dot product does not yield exactly zero but a smallremainder in the order of one element of the sequence. One example ofpseudo-orthogonal sequences is two m-sequence which are shifted againsteach other. A m-sequence is a pseudonoise sequence known in the art withspecific autocorrelation properties. M-Sequences can be generated byusing a feedback shift register. Thus, as used herein, the term“orthogonal” is to be interpreted as including both “pure orthogonal”and “pseudo-orthogonal”.

The above described SYNC symbol sequences provided by modulation(multiplication) of a predetermined SYNC symbols with orthogonalsequences may be regarded as a representation of orthogonal sequencesweighted by the predetermined symbol. This means that the orthogonalsequences, for example rows or columns of a Hadamard matrix, aremultiplied for each subcarrier by the respective complex constellationvector (constellation point) of the predetermined SYNC symbol. UsingHadamard sequences as orthogonal sequences, the predetermined or regularSYNC symbol is multiplied at each SYNC transmission position either with+1 or −1 based on the value of the orthogonal sequence elementcorresponding to the number of the transmission position.

The predetermined complex SYNC symbol may in an embodiment be selectedfor each subcarrier from the 4 constellation points of a 4-QAMmodulation representing the bit sequences 00, 01, 10 and 11. Thepredetermined SYNC symbols can be different for each subcarrier.

It is to be noted that the SYNC symbol sequences transmitted on allchannels of the vector transmission system are made orthogonal (pureorthogonal or pseudo-orthogonal) by multiplying the predetermined SYNCsymbol with the orthogonal sequences and are therefore statisticallyindependent. This allows using the SYNC symbol sequences as pilotsignals as well as probing signals for fast estimation or acquisition ofthe FEXT coupling coefficients between the channels for example when anew line joins the vectored group or a fast updating of the coefficientsof the FEXT cancellation matrix in an already existing vectored group.

FIG. 5 shows an embodiment of the transmission of orthogonal modulatedSYNC symbol sequences in vectored DSL channels. As can be seen, the SYNCsymbols for each orthogonal SYNC symbol sequence is transmitted on eachchannel (line) at the same time, i.e. at the same time slot. Between twoconsecutive SYNC symbols, data symbols are transmitted. In oneembodiment of a VDSL system, the number of data symbols betweenconsecutive SYNC symbols may be 256. The data symbols and the SYNCsymbols form then a structure known as a superframe.

In view of the above, in FIG. 5 the symbol sequence OS11 . . . OS1 n ofChannel 1 may be therefore be the result of the multiplication of afirst row of the Hadmard matrix with a predetermined SYNC symbol u0. Thesymbol sequence OS21 . . . OS2 n may be the result of the multiplicationof a second row of the Hadamard matrix with the predetermined SYNCsymbol u0 etc. For each channel the overall symbol sequence may thenobtained by periodically repeating the respected symbol sequenceobtained by multiplication of the respective row of the Hadamard matrixwith the predetermined SYNC symbol u0.

In order to estimate or determine the transmission matrix H, a slicererror is determined at the respective receiver side based on thereceived signal. For determining the cross-talk coefficients of matrix Hfor upstream communication frequencies (subcarriers), the transceivers104 at the Central Office side have to perform the slicer errormeasurement while for determining the matrix coefficients for downstreamcommunication frequencies (subcarriers), each of the transceivers 108 atthe respective CPE sides have to perform the slicer error measurement.The corresponding receiver units are therefore operable to receive asequence of signals and to determine a slicer error by comparing thesequence of received signals with a reference. The reference for thereceived sequence of symbols is the expected sequence of SYNC symbolswhich is equal to the transmitted SYNC symbol sequence. Updating of thecross-talk coefficients H_(i,j) is then based on the measured slicererror.

In more detail, each transceiver unit measures each of the cross-talkprobe signals (SYNC symbol signal) received and demodulates the signalby equalizing the signal and Fourier transforming the equalized signal.Equalizing the signal provides compensation for the signal attenuationon the respective channel represented by the diagonal coefficients ofmatrix H. Finally, a received complex number (symbol in theconstellation space) is obtained. Due to the cross-talk effects and thealien noise experienced during the transmission, the received complexnumber and the original send complex number in the constellation spacedeviate from each other.

The coefficients of Matrix H(k) are then determined from the measuredreceive signal by estimating a receive error (which will be alsoreferred to as slicer error) of the receive signal Y which indicates adeviation of the received signal from an expected constellation point.The slicer error may in some embodiments only include the sign of thedifference between the original transmitted and the received symbolsignal as outlined below.

If the cross-talk probe signals, i.e. the orthogonal sequence of symbolsis transmitted at a time t1 on all of a plurality of L channels arerepresented by a sender vector

${\overset{\rightarrow}{x}\left( {t\; 1} \right)} = \begin{bmatrix}{x_{1}\left( {t\; 1} \right)} \\{x_{2}\left( {t\; 1} \right)} \\\ldots \\{x_{L}\left( {t\; 1} \right)}\end{bmatrix}^{T}$

where T indicates the transposed vector then the sequence of thecross-coupling probe signals transmitted at time slots t1, t2, . . . tNon all of the plurality of L channels can be obtained by a matrix

$X = \begin{bmatrix}{x_{1}\left( {t\; 1} \right)} & {x_{1}\left( {t\; 2} \right)} & \ldots & {x_{1}({tN})} \\{x_{2}\left( {t\; 1} \right)} & {x_{2}\left( {t\; 2} \right)} & \ldots & {x_{2}({tN})} \\\ldots & \ldots & \ldots & \ldots \\{x_{L}\left( {t\; 1} \right)} & {x_{L}\left( {t\; 2} \right)} & \ldots & {x_{L}({tN})}\end{bmatrix}^{T}$

where T indicates a transposed matrix. Similar, the sequence received atthe receiver on all of the L channels at the time slots t1, t2, . . . tNcan be written as a matrix

$Y = {\begin{bmatrix}{y_{1}\left( {t\; 1} \right)} & {y_{1}\left( {t\; 2} \right)} & \ldots & {y_{1}({tN})} \\{y_{2}\left( {t\; 1} \right)} & {y_{2}\left( {t\; 2} \right)} & \ldots & {y_{2}({tN})} \\\ldots & \ldots & \ldots & \ldots \\{y_{L}\left( {t\; 1} \right)} & {y_{L}\left( {t\; 2} \right)} & \ldots & {y_{L}({tN})}\end{bmatrix}^{T}.}$

The receive matrix Y can be obtained by multiplying the sender Matrix Xwith the transmission matrix H and adding a matrix Δ which takes intoaccount the alien noise added during the transmission: Y=XH+Δ.

The matrix representing the equalized receive error can then beexpressed by Z=Y(H_(d))⁻¹−X=XH(H_(d))⁻¹−X+Δ(H_(d))⁻¹=XF+Δ(H_(d))⁻¹ whereH_(d) ⁻¹ represents a diagonal matrix having as coefficients thediagonal coefficients H_(ii) of matrix H which is sometimes referred toas frequency equalizer (Feq) and F represents the equalizedcross-coupling matrix without the transmit coefficients H_(ii), i.e. alldiagonals are zero.

Assuming a uniform distribution of the alien noise and using a leastsquare estimate, the estimate {circumflex over (F)} of thecross-coupling matrix H can be expressed by

{circumflex over (F)}−(X* ^(T) X)⁻¹ X* ^(T) Z  [Equation 1]

where X*^(T) represent the transposed and complex conjugated matrix ofmatrix X and (X*^(T)X)⁻¹ is the inverse of the autocorrelation matrix(X^(*T)X).

By using orthogonal sequences as described above, the estimatedcross-talk matrix can be calculated in a simple way since (X*^(T)X)⁻¹ isfor a modulation with orthogonal sequences proportional to the unitymatrix and convergence of the cross-talk is reached faster and easier.For example, assuming that X=u₀S, where S is an orthogonalWalsh-Hadamard matrix and u₀ is a predetermined SYNC symbol, (X*^(T)X)⁻¹becomes equal to |u₀|²(S^(T)S)⁻¹.

While for some operation it may be usefull to determine for eachtransmitted SYNC symbol a slicer error and feed the slicer error ascross-talk update information to the management unit 112 for updatingthe cross-talk coefficients of matrix H, it may be usefull to have alsooperation modes with selectable lower rates of updating the cross-talkcoefficients. For example, while it may be usefull to provide a trainingwith a high update rate in situations when the coefficients have to bequickly learned for example during a start of the vector transmissionsystem or during a line joining of a new line (a new line startsoperation), it may be usefull after a successful learning of thecoefficients to provide lower update rates for tracking the coefficientsduring data mode. Such an operation mode may for example include but notlimited to a so called tracking mode. Embodiments described in thefollowing provide a concept allowing a flexible update rate while stillhaving the benefit of the easy calculations as described above.

The embodiments presented below use orthogonal symbol sequences ascross-talk probe signals and lower or change an update rate ofcross-talk coefficients without changing the rate at which theorthogonal symbols are transmitted. Rather than changing the rate atwhich the probe symbols are transmitted, in the concept described belowa carefull selection of a subsequence of the transmitted orthogonalsymbols is performed by using a predetermined selection scheme. With thepredetermined selection scheme, it is possible to construct from thetransmitted orthogonal sequence of symbols a subsampled sequence ofsymbols at a lower rate. With the predetermined selection scheme, thesubsampled sequence of symbols are also obtained again to be anorthogonal sequence. The rate of update information can therefore belowered or changed while still providing always a sequence of updateinformation to the management unit which is derived from an orthogonalsequence, i.e. the subsampled sequence.

A general flow diagram 200 according to one embodiment will be explainednow with respect to FIG. 2.

At 202, a sequence of symbols is transmitted in a vector transmissionsystem from a first transceiver to a second transceiver, the sequence ofsymbols being based on a modulation of a predetermined symbol with anorthogonal sequence. The orthogonal sequence used for each channel ofthe vector transmission system is one sequence of a set of mutuallyorthogonal sequences. In embodiments, the transmitted symbol sequence isobtained by providing for each channel an orthogonal symbol sequence andrepeatedly transmitting the orthogonal symbol sequence over therespective channels. The orthogonal symbol sequence is obtained bymultiplying an orthogonal sequence with a predetermined symbol, i.e.{right arrow over (x)}(t1)=(x₁(t1), x₁(t2), . . . x_(N)(tN))=u₀□(s₁(t1),s₁(t2), . . . s_(N)(tN))=u₀s₁(t1), u₀s₁(t2), . . . u₀s_(N)(tN)). Thetransmitted symbol sequence represents in embodiments a pilot symbolsequence transmitted at predetermined synchronization time slots.Between two successively transmitted symbols of the sequence, othersymbols representing user data may be transmitted.

At 204, symbols are selected from the sequence of transmitted symbolsfor providing cross-talk update information based on a transmissionerror of the selected symbols. The selected symbols are a subset of thetransmitted symbols, i.e. only a part of the transmitted symbols areselected. As will be described in more detail below, in embodiments theselection is provided by a predetermined scheme such that the sequenceof the selected symbols is again an orthogonal sequence. Thus, whenusing the same scheme for each channel, the sequence of selected symbolsfor one channel is orthogonal to the sequence of selected symbols oneach other channel.

In one embodiment, the selection is a contiunous selection of every kthsymbol wherein k is an odd integer which may be selected and thereaftermaintained. The selection is provided continuously over the sequence atleast until as many symbols are selected number as the size N of theorthogonal sequence is. The subsampling may extend however much longerthan until the N symbols are selected. In one embodiment, the size N ofthe orthogonal sequence is equal to 2^(n) where n is an integer number.In one embodiment, the orthogonal sequence is a pseudo-orthogonalsequence of size 2^(n)−1.

Cross-talk update information may be regarded as information which isprovided for updating a FEXT cross-talk coefficient for example whenlearning or tracking cross-talk coefficients of the vector transmissionsystem. Typically, when updating a cross-talk coefficient, an oldcross-talk coefficient is replaced by a new cross-talk coefficient. Thenew cross-talk coefficient may be calculated by using techniques such asLMS (least mean square) or other updating techniques known in the art.The cross-talk update information may be in embodiments the slicer-errorof the transmitted symbol. Slicer error may be the difference vector ofthe original transmitted symbol and the received symbol or only the signof the trans-mission error (sign of real part and/or sign of imaginarypart of the error).

At 206, cross-talk update information is provided based on atransmission error of a respective selected symbol received at thesecond transceiver. By selecting the symbols, only the transmissionerror of the selected symbols is taken into account for updating whilethe transmission error occurred for non-selected symbols is not takeninto account when updating the FEXT coefficients. Therefore, thebandwidth required for transmitting the update information and theamount of computation required for updating is reduced compared to theoperation mode in which update information is provided based on thetransmission error for each transmitted symbol.

In the following, more detailed embodiments will be described based onusing Hadamard sequences as orthogonal sequences. However, it should beunderstood that the embodiments described herein are not limited tothese type of orthogonal sequences and other orthogonal sequences may beused as well.

As described above, a Hadamard sequence is a sequence obtained from arow (or a column) of a Hadamard matrix. For sizes N that are powers oftwo, i.e. N=2^(n), the Hadamard matrix can be constructed as follows:

$H_{2} = \begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ $H_{2n} = \begin{bmatrix}H_{n} & H_{n} \\H_{n} & {- H_{n}}\end{bmatrix}$

The characteristics of Hadamard matrixes are:

H_(N) ^(T)=H, where H_(n) ^(T) is the transposed matrix andH_(N)·H_(N)=N·I(N) where I(N) is the Identity matrix.

For illustration purposes, FIG. 3 a shows an example of a Hadamardmatrix of size N=8. It can be easily seen that each row of the Hadamardmatrix is orthogonal to each other row as well as each column isorthogonal to each other column.

FIG. 3 b shows an embodiment of constructing pilot symbol sequences for6 channels based on the Hadamard matrix of size 8. The pilot symbolsequences may for example be used as SYNC symbol sequences in asuperframe structure as described with respect to FIG. 5. In order toconstruct an orthogonal sequence, 6 Hadamard sequences (either rows orcolumns) are selected from the Hadamard matrix. In the exemplaryembodiment of FIG. 3 b, the columns 2 to 7 have been choosen asorthogonal basis for the channels 1 to 6. On each channel, after oneHadamard sequence is completed, the Hadamard sequence for the respectiveline is repeated periodically. It is to be noted that FIG. 3 b shows theorthogonal sequence of the Hadamard symbols having values of 1 or −1. Asdescribed above, for providing a symbol sequence based on the orthogonalsequence, each orthogonal sequence is multiplied with a predeterminedcomplex symbol u₀.

In order to provide a reduced update rate, update information isgenerated based on a transmission error only for a subsampled sequenceof the transmitted symbol sequence.

Embodiments described herein use predetermined selection schemes(subsampling schemes) which allow to regularly subsample the transmittedsymbol sequences such that the subsampled sequences are again orthogonalsequences. In other words, a subsampled sequence Sub-S1 is orthogonal toeach other subsampled sequence Sub-S2, Sub-S3 . . . of the otherchannels of the vector transmission system and fulfills the identityrequirement when performing a multiplication with itself.

In the following, it will be shown that continuously subsampling each ofa set of orthogonal sequences of size 2^(n) every k^(th) symbol willresult in a set of sub-sampled sequences which are orthogonal if k is anodd integer number. Contrary, if k is an integer even number, the set ofsub-sampled sequences are not obtained to be orthogonal to each otherwhen subsampling every k^(th) symbol is provided continously.

In the following it is first shown that a change of the orderconsistently performed in all sequences will not affect theorthogonality while a replacement of one by another will not maintainthe orthogonality. As outlined above, the dot product of two sequencesS′ and S″ is obtained by s′(1)·s″(1)+s′(2)·s″(2)+s′(3)·s″(3)+s′(4)·s″(4). . . +s′(N)·s″(N). Exchanging the sequence order consistently in bothsequences will not affect the dot product. For example exchanging s′(2)with s′(3) and s″(2) with s″(3) in sequences S′ and S″ will not affectthe dot product since in both cases the sum s′(2)·s″(2)+s′(3)·s″(3) isthe same. However, when replacing at least one of the sequence valuewith another sequence value, i.e. repeating one of the sequence numbersat least twice while ommiting one sequence value, orthogonality can ingeneral not be maintained. For example, if instead of exchanging number2 and number 3 consistently in all sequences, the value of sequencenumber 3 is replaced consistently for all orthogonal sequences by thevalue of sequence number 2 such that the sequence reads s(1), s(2),s(2), s(4) . . . , then the dot product would reads′(1)·s″(1)+s′(2)·s″(2)+s′(2)·s″(2)+s′(4)·s″(4) . . . +s′(N)·s″(N). Ifthe replacement is consistently for all sequences of the set oforthogonal sequences, then orthogonality for all sequences of the set oforthogonal sequences can not be maintained in general.

In the following, it is assumed that a set of sequences is constructedby periodically repeating a resepective orthogonal sequence having sizeN. From the above it can be concluded that in subsampling each sequenceof such a set of sequences with the same predetermined rule,orthogonality of the subsampled sequences is only maintained when thesequence numbers of the orthogonal sequences do not repeat until Nsymbols are selected (sampled). Or in other words, when selecting thesubsampled symbols from the sequence N times, all of the N differentsymbols (symbols numbers) of the orthogonal sequence (being the basisfor constructing the sequence) have to appear once in the N subsampledsequence numbers.

For a set of sequences constructed by regularly repeating respectiveorthogonal sequences of size 2^(n) _(r) it will be showed in thefollowing that a continously regular subsampling of every k^(th)sequence value (i.e. a subsampling where the number of samples betweentwo subsampled numbers is maintained constant to (k−1)) only maintainsorthogonality if k is an odd value. In other words, when continousregular sampling is provided, only subsampling every odd sequence value(such as every 3^(rd) or every 5^(th),etc.) maintains orthogonalitywhile subsampling every even sequence value (such as every 2^(nd), orevery 4^(th), etc) will not preserve orthogonality of the sequences. Inorder to show the validity of this rule one can start with reformulatingthe requirement for orthognality of the subsampled sequence in view ofthe above as follows. Assuming that in the subsampled sequences one ofthe subsampled sequence numbers of the sequence repeats the first timeafter m subsampled values, then the subsampled sequence is orthogonal ifm is equal to the size of the orthogonal sequence N=2^(n). Or, in a moredescriptive way, starting from 0, one may determine the integer number mindicating how many times it is needed to add a number k until one endsup at a same orthogonal sequence number (Haddamad sequence number) inthe sequence.

If m is equal to the size N=2′, then the subsampled sequence ismaintained to be orthogonal. If it is lower than N=2′, then thesubsampled sequence will not be orthogonal. Since the same orthogonalsequence number repeats in the sequence after one orthogonal sequencehas been completed which is every N=2^(n) value, it is equal to findnatural numbers p, m satisfying

p·2^(n) =m·k p,n,m,kε□  Eq. 1

and determine whether the minimial value for m which allows to solveequation 1 is equal to 2^(n).

Here, p indicates how many times the orthogonal sequence (Haddamardsequence) has been repeated until the first repetition occurs and mindicates the number which is required to obtain the same sequencenumber for a subsampling with a factor of k. If the minimimal value form is equal to m_(min)=2^(n), then the subsampeld sequence is orthogonal,if m_(min)<2^(n) then it is not orthogonal.

Equation 1 can be written in the form

$\begin{matrix}{\frac{m}{p} = {\frac{2^{n}}{k}.}} & {{Eq}.\mspace{14mu} 2}\end{matrix}$

Assuming now k to be an even number, i.e. k can be written as k=2i, thenEquation 2 can be written as

$\begin{matrix}{\frac{m}{p} = {\frac{2^{n}}{2 \cdot i} = \frac{2^{n - 1}}{i}}} & {{Eq}.\mspace{14mu} 3}\end{matrix}$

By observing Eq. 3 one obtains for m:

$m = {{\frac{2^{n}}{2 \cdot i}p} = {\frac{2^{n - 1}}{i}{p.}}}$

For taking the integer numbers p and i to be equal, on can realize thatm=2^(n-1) is a solution to equation 1. Therefore, m_(min)<2^(n) is validfor even integer numbers k and the subsampled sequence is therefore notobtained orthogonal.

On the other hand, starting from Eq. 2 and assuming k to be an integerodd value, one can write k=2i+1, i being an integer. Eq. 2 can thereforebe written as

$\begin{matrix}{\frac{m}{p} = \frac{2^{n}}{{2 \cdot i} + 1}} & {{Eq}.\mspace{14mu} 4}\end{matrix}$

Solving Eq. 4 for m results in:

$\begin{matrix}{m = {\frac{2^{n}}{{2i} + 1}{p.}}} & {{Eq}.\mspace{14mu} 5}\end{matrix}$

In Eq. 5, since 2^(n) divided by (2i+1) is not an integer, it isrequired that p divided by (2i+1) is an integer in order to allow m tobe an integer. The lowest possible value for m is therefore obtainedwhen

${\frac{p}{{2i} + 1} = 1},$

yielding as minimum m_(min)=2^(n). Since this is the requirement fororthogonality it is shown that a continuous regular subsampling everyk^(th) element of the sequence is orthogonal if k is an odd integervalue.

In view of the above, one embodiment 400 of providing an updating ofcross-talk coefficients in a vector transmission system is shown in FIG.4 a. In FIG. 4 a, at 402 a set of orthogonal symbol sequences isgenerated by multiplying a set of orthogonal sequences of size N=2^(n)with a predetermined symbol. As outlined above, n is here an integervalue and the predetermined symbol can be any symbol for example a 4-QAMsymbol. At 404, a symbol sequence is transmitted over a channel of thevector transmission system by repeatedly transmitting one orthogonalsymbol sequence of the set of orthogonal symbol sequences. It is to benoted here that in addition to the one symbol sequence other symbolsequences are transmitted over other channels of the vector transmissionsystem by repeatedly transmitting on a respective channel an orthogonalsymbol sequence of the set of orthogonal symbol sequences as is shownfor example in FIG. 3 a. The repeatedly transmitted orthogonal symbolsequences is different for each channel since otherwise the requirementthat an orthogonal symbol sequence transmitted on one channel isorthogonal to an orthogonal symbol sequence on another channel is notfulfilled.

At 406, a regularly subsampling is performed by selecting every k^(th)received symbol (sampling factor k). Here k is allowed to be aselectable but odd integer value. In embodiments, k may be allowed tovary dependent on the requirements. The value of k may be in someembodiments be determined by a central unit such as the management unitshown in FIG. 1 a.

At 408, a slicer error is provided for each of the selected (regularysubsampled) symbols received at the receiver. As outlined above, aslicer error may in embodiments include the full transmission error,i.e. a complex vector of any size, or may include only the sign of thetransmission error.

At 410, cross-talk coefficients are updated based on the slicer error ofeach of the selected symbols. In one embodiment, the slicer error may betransmitted back from the receiver to the transmitter side. For example,in order to compensate FEXT in downstream direction, each of the secondtransceivers units 108 shown in FIG. 1 a may determine the slicer errorand transmit the slicer error back to the management unit for allowingupdating of coefficients stored in the management entity 112. It is tobe noted that in one embodiment the slicer may be immediatelytransmitted back. A dedicated channel may be used to transmit the slicererror back. In one embodiment, the slicer error is transmitted back assoon as the dedicated channel is free to transmit. In anotherembodiment, for each channel, slicer errors for a number of selected maybe stored for a short time at the receiver side and then transmittedback together.

In some embodiments, no transmission over the vectored transmissionsystem may be required for updating the cross-talk coefficients based onthe slicer error. For example, for updating the FEXT coefficients inupstream direction, each of the first transceiver determines arespective slicer error as outlined above. In this case no transmissionover the respective channels of the vector communication system isrequired since the first transceivers are located at the same side asthe management entity 112 which performs the updating of thecoefficients. It is to be noted that in the described embodiments, thereceiver alone may determine the sampling factor k without informing thetransceiver at the other side.

According to one embodiment, a subsampling factor may be predeterminedor determined for example based on a desired or required update rate. Inan operation mode where only odd subsampling factors are allowed inorder to provide an orthogonal subsampled sequence, a desired orrequired subsampling factor may be calculated based on the desired orrequired update rate. If the desired or required subsampling factor isnot obtained to be an odd integer, then the next lowest odd integer orthe next higher odd integer may be selected.

In embodiments, a change of the update rate, i.e. a change of thesubsampling factor may be provided on the fly.

According to one embodiment, a central unit such as the management unit112 may determine the subsampling factor k for each vectored channel.The subsampling factor k is then transferred together with a startingindex indicating the first symbol to be selected with the subsamplingfactor k to the corresponding transceivers, i.e. either transceivers 104or transceivers 108 depending whether the subsampling factor kcorresponds to upstream FEXT probing or downstream FEXT probing.

Having described now an embodiment for the size N=2^(n) of obtaining anorthogonal sequence by continuously subsampling every k^(th) sample withk being only allowed to be an integer, a further embodiment will bedescribed for providing a set of subsampled orthogonal sequences byselecting symbols from each sequence based on a predetermined scheme.

For sampling with even integer factors p (for example 2, 4, 6 etc), ashift by one in the subsampling is required after a predetermined numberof subsampled symbols as will be outlined below.

Starting with the above described requirement for orthogonality that thefirst repeated orthogonal sequence number of the subsampled symbols hasto occur after N=2^(n) subsampled symbols where N is the size of eachorthogonal sequence, one can first determined the number of times asubsampling can occur with an even integer sampling factor k until thefirst repetition occurs and then shift the next symbol to be sampled byone. Since k is an even number, k can be written as k=k1·2^(n1) with k1being an odd integer and n1 being an integer.

Inserting the above into Eq. 2, one obtains

$\begin{matrix}{m = {\frac{2^{({n - {n\; 1}})} \cdot p}{k\; 1}.}} & {{Eq}.\mspace{14mu} 6}\end{matrix}$

Since k1 is an odd integer and therefore not dividable by 2 withoutremainder, the smallest integer number for m is obtained when p is equalto k1 since then

$\frac{p}{k\; 1} = 1.$

One then observes that the smallest number of m is m_(min)=2^((n-n1)).In other words, having determined the number n1, one can calculate thenumber m_(min)=2^((n-n1)) after which the first repetition will occur.Having then sampled m_(min)=2^((n-n1)) the next symbol to be regularlysubsampled sequence is then shifted by one and thereafter againregularly subsampled with the even sampling factor 2^((n-n1)) etc. Thesubsampling of 2^((n-n1)) sequence symbols with subsampling factor k andthen shifting the next symbol to be regularly subsampled by one has tobe performed 2^(n1) times until all 2^(n) subsampled symbols of anorthogonal subsampled sequence are obtained.

FIG. 3 d shows for illustration purposes an embodiment implementing theabove described scheme for a set of sequences based on the Hadamardmatrix shown in FIG. 3 a. In FIG. 3 d, assuming a sampling factor ofk=2, one determines in view of the above that n1=1 and therefore a shiftwill be performed after 2²=4 subsampled symbols. Thus, after subsampling4 symbols, instead of providing the next regular symbol with sequencenumber 8, a shift of one symbol is provided such that the next sampledsymbol is 8+1=9.

It can be easily seen in FIG. 3 d that by providing subsampling with theabove scheme, a subsampled sequence for one channel is orthogonal to asubsampled sequence of each other channel.

FIG. 4 b shows a flow diagram illustrating an embodiment 420 forproviding an updating of cross-talk coefficients in a vectortransmission system implementing the above described subsampling schemefor even integer subsampling factors.

Starting at 422, a set of orthogonal symbol sequences is generated bymultiplying a predetermined symbol with a set of orthogonal sequences ofsize N=2^(n).

At 424, a symbol sequence is transmitted over a channel of a vectortransmission system by repeatedly transmitting one orthogonal symbolsequence of the set of orthogonal symbol sequences. It is to beunderstood that other symbol sequences of the set of symbol sequencesmay be repeatedly transmitted on other channels of the vectortransmission system.

At 426, a subsampling counter value is set to 0. At 428 a startingsequence number s is selected and the symbol having the startingsequence number is selected (subsampled).

At 430, the subsampling counter value indicating the number of selectedsymbols is increased by one.

At 432, it is determined whether the subsampling counter value exceeds apredetermined threshold. The predetermined threshold may be calculatedby determining the integer number k1 which is the number of times, thesubsampling factor k contains the factor 2 as described above.

If it is decided in 432 that the predetermined threshold is notexceeded, the flow diagrams continues at 434 by increasing the selectionnumber s for the next symbol by the subsampling factor k. Then at 436,the symbol with sequence number s is selected and at 438 a slicer erroris provided for the selected symbol. At 440, a cross-talk coefficient isupdated based on the determined slicer error of the selected symbol.

The flow diagram then moves back to 430 where the subsampling countervalue is increased again by one and thereafter to 432 where it is againdetermined whether the subsampling counter value exceeds thepredetermined threshold.

If it is finally determined in 432 that the predetermined threshold isexceeded, the selection number s for the next symbol is increased by(k+1) which provides the shift by one symbol in the subsampling.Thereafter, the subsampling counter value is set to 0 and the processcontinues with 436, 438 and 440 as described above.

In the above, it is assumed to shift the subsampling of the next symbolby +1 compared to the expected regular sampled symbol. However, in otherembodiments, other shifts by an odd number may be encompassed.

It is to be noted that the above described embodiment is one embodimentof implementing the selecting of every k-th symbol from the sequence oftransmitted symbols, wherein k is an even integer number such that afterselecting in the subsampling a predetermined number of symbols the nextsymbol is selected as the (k+1)th symbol counted from the previousselected symbol.

It is to be noted here that the scheme of continuously subsampling withodd subsampling factors without shifting can be combined with thesubsampling with even subsampling factors and providing a shift. Forexample, in one embodiment, one of the above described schemes may beselected. Selection may for example be based on a required or desired orexpected update rate. Since the higher the subsampling factor k is, thelower is the actual difference in the update rate for two subsequentfactors k and k+1. Thus, the difference between the update rate betweentwo subsequent odd and even values k and k+1 is small. Therefore, in oneembodiment, when the subsampling factor is over a predeterminedthreshold, only an odd sampling factor may be selected by looking forthe next appropriate odd value k.

Having described now embodiments wherein the size N of the orthogonalsequence is 2^(n), in the following an embodiment having a sizeN=2^(n)−1 will be described. As an example, sequence with size N=2^(n)−1include m-sequences which are constructed to have this size.

For orthogonal sequence of size N=2^(n)−1, Eq. 1 may be modified toobtain:

p·(2^(n)−1)=m·k p,n,m,kε□  Eq. 7

Assuming k to be an even integer, one can write k=2i and easily observethat

$m = {\frac{2^{n} - 1}{2 \cdot i}{p.}}$

Since 2^(n)−1 is an odd number, 2^(n)−1 can not be divided by theinteger number 2i. Therefore the requirement is that

$\frac{p}{2 \cdot i}$

is an integer number. The smallest possible m is obtained

$\frac{p}{2 \cdot i} = 1$

for which m is obtained to be m=2^(n)−1 showing that the subsamplingwith an even subsampling factor k results in an orthogonal subsampledsequence.

Assuming k to be an odd integer, k can be written as k=2i+1. Equation 7can then be reformulated to obtain

$m = {\frac{2^{n} - 1}{{2 \cdot i} - 1}p}$

In many cases,

$\frac{2^{n} - 1}{{2 \cdot i} - 1}$

will not be an integer and therefore

$\frac{p}{{2 \cdot i} - 1}$

is requested to be an integer in order to determine valid solutions forEq. 7. In these cases, similar to the above, the smallest number for mis determined by

$\frac{p}{{2 \cdot i} - 1} = 1$

which yields as minimum m_(min)=2^(n)−1 which shows that the subsampledsequence is orthogonal. For example for k>N, i.e. for i>2^(n 1),

$\frac{2^{n} - 1}{{2 \cdot i} - 1}$

can not be an integer. Thus, for k>N and k being an odd integer, thesubsampled sequence is always an orthogonal sequence.

Cases in which

$\frac{2^{n} - 1}{{2 \cdot i} - 1}$

is an integer can be determined by observing

$\frac{2^{n} - 1}{{2 \cdot i} - 1} = {\frac{2^{n}}{{2 \cdot i} - 1} - {\frac{1}{{2 \cdot i} - 1}.}}$

Since the later term can not be an integer except for the trivial caseof i=1 (k=1, i.e. sampling every symbol), in order to have

$\frac{2^{n}}{{2 \cdot i} - 1} - \frac{1}{{2 \cdot i} - 1}$

being an integer the following must apply:

2^(n) mod (2i−1)=2^(n) mod (k)=1.  Eq. 8

These cases are rare. If Eq. 8 would apply, the sequence would repeatafter m_(min) symbols, wherein m_(min) can be determined for p=1 to be

$m_{\min} = {\frac{N}{k} = {\frac{2^{n} - 1}{{2\; i} - 1}.}}$

In case n and k are desired such that Eq. 8 would be valid, according toone embodiment an even integer k+1 or k−1 next to k may be selected toobtain an orthogonal sequence. In another embodiment, if Eq. 8 would bevalid, one may use the scheme of shifting one symbol (or other numbersof symbols) after

$m_{\min} = \frac{N}{k}$

symbols are selected as described with respect to FIGS. 3 d and 4 b.

Taking in view of the described embodiments a system into accountwherein the orthogonal sequences are allowed to be either Haddamadsequences which always have a size of N=2^(n) as well as m-sequenceswhich always have a size of N=2^(n)−1, it is to be noted that regularlyand continuously subsampling with an odd sampling factor k may allow forboth cases to obtain subsampled sequences which are orthogonal exceptfor the rare cases outlined above for N=2^(n)−1 fulfilling k<N and 2^(n)mod (k)=1.

Subsampling regularly and continously with a subsampling factor k beingan even integer number, it can be seen that for N=2^(n)−1 alwaysorthogonal sequences are obtained while for N=2^(n) no orthogonalsequences are obtained and the scheme of shifting by one (or otherpredetermined numbers of symbols) may be used as outlined with respectto FIGS. 3 d and 4 b.

In view of the above, according to an embodiment, the size N of theorthogonal symbol sequence may be determined and the continous andregular selecting of every k-th symbol from the sequence of transmittedsymbols until at least N symbols are selected may be performedthereafter. If the size N is determined to be 2^(n)−1, k is selectablefrom an odd or even integer number. If the size N of the orthogonalsymbol sequence is determined to be 2^(n)−1 with n being an integernumber, the regular continous subsampling with subsampling factor k mayonly be performed when k is an odd integer. Then k is an integer numberselectable only from odd integer numbers. Here, if k is required ordesired to be an even number, either the next odd integer number may bechosen or the above shifting scheme may be performed.

In other words, according to one embodiment, by determining a size N ofthe orthogonal sequence and selecting a desired or requested subsamplingfactor k, the continous regular sampling with subsampling factor k(continously subsampling every kth symbol) is only performed it has beendetermined or calculated for the selected subsampling factor k and thedetermined size N that for each symbol of the orthogonal symbolsequence, after selecting said symbol all other symbols of theorthogonal symbol sequence are selected before said symbol is againselected. Or in other words, N subsequently selected symbols contain allsymbols (all symbol numbers) of the orthogonal symbol sequence. If thisis not the case, a new subsampling factor k may be selected according toone embodiment. According to another embodiment a non-continoussubsampling with the shifting scheme described with respect to FIGS. 3 dand 4 b may be selected.

For DMT systems, since the subcarriers are independent, the abovedescribed updating may in embodiments be provided independently for eachsubcarrier.

Referring now to FIG. 6, an embodiment of a device 600 which may beimplemented in a transceiver such as any of the transceivers 104 and 108in order to allow a selection of symbols for cross-talk updating will bedescribed. The device 600 includes an input 602 configured to receive asequence of symbols transmitted in a vector transmission system by afirst transceiver of the vector transmission system. As described above,the sequence of symbols is based on a modulation of a predeterminedsymbol with an orthogonal sequence. For example, the sequence of symbolsmay be generated by repeatedly transmitting an orthogonal sequence whichis constructed by a multiplication of a predetermined symbol with anorthogonal sequence. The device further comprises a selection circuit604 configured to select a subset of the received symbols of thesequence as described above. A slicer 606 is provided in the device 600and is configured to generate a slicer error for each symbol of thesubset selected by the selection circuit. Device 600 further includes acircuit 608 configured to transmit cross-talk update information to amanagement unit 112. The cross-talk update information may be the slicererror or may be other information derived from the slicer error. Inembodiments, the cross-talk update information may be transmitted to themanagement unit over a channel of the vector trans-mission system. Inanother embodiment, when the device 600 is implemented on the same sideas the management unit, other transmission paths or transmission linessuch as cables or metal lines connecting the transceivers 104 with themanagement unit may be used.

A further embodiment of a vector transmission device 700 will now bedescribed with respect to FIG. 7. Device 700 includes a symbol generator702 configured to generate the symbol sequence based on a modulation ofa predetermined symbol with an orthogonal sequence as described above.As indicated in FIG. 7, the symbol generator 702 may be provided in oneof the transceivers 104 shown in FIG. 1. Device 700 further includes aselection circuit 704 configured to provide selection informationindicating which symbols of the sequence of symbols are to be used forupdating cross-talk coefficients and which symbols of the sequence ofsymbols are not to be used for updating cross-talk coefficients. Theselection information is provided to the transceivers 104 and/or to thetransceivers 112 (which are not shown in FIG. 7) for allowing thetransceivers to provide a selection of the received symbols as describedabove.

In the above description, embodiments have been shown and describedherein enabling those skilled in the art in sufficient detail topractice the teachings disclosed herein. Other embodiments may beutilized and derived there from, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure.

This Detailed Description, therefore, is not to be taken in a limitingsense, and the scope of various embodiments is defined only by theappended claims, along with the full range of equivalents to which suchclaims are entitled.

Such embodiments of the inventive subject matter may be referred toherein, individually and/or collectively, by the term “invention” merelyfor convenience and without intending to voluntarily limit the scope ofthis application to any single invention or inventive concept if morethan one is in fact disclosed. Thus, although specific embodiments havebeen illustrated and described herein, it should be appreciated that anyarrangement calculated to achieve the same purpose may be substitutedfor the specific embodiments shown. This disclosure is intended to coverany and all adaptations or variations of various embodiments.Combinations of the above embodiments, and other embodiments notspecifically described herein, will be apparent to those of skill in theart upon reviewing the above description.

It is further to be noted that specific terms used in the descriptionand claims may be interpreted in a very broad sense. For example, theterms “circuit” or “circuitry” used herein are to be interpreted in asense not only including hardware but also software, firmware or anycombinations thereof. The term “data” may be interpreted to include anyform of representation such as an analog signal representation, adigital signal representation, a modulation onto carrier signals etc.Furthermore the terms “coupled” or “connected” may be interpreted in abroad sense not only covering direct but also indirect coupling.

It is further to be noted that embodiments described in combination withspecific entities may in addition to an implementation in these entityalso include one or more implementations in one or more sub-entities orsub-divisions of said described entity. For example, specificembodiments described herein described herein to be implemented in atransmitter, receiver or transceiver may be implemented in sub-entitiessuch as a chip or a circuit provided in such an entity.

The accompanying drawings that form a part hereof show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of the technical disclosure. It is submitted withthe understanding that it will not be used to interpret or limit thescope or meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, where each claim may standon its own as a separate embodiment. While each claim may stand on itsown as a separate embodiment, it is to be noted that—although adependent claim may refer in the claims to a specific combination withone or more other claims—other embodiments may also include acombination of the dependent claim with the subject matter of each otherdependent claim. Such combinations are proposed herein unless it isstated that a specific combination is not intended.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by devices having features such asmeans or circuits configured to perform each of the respective steps ofthese methods.

1.-25. (canceled)
 26. A method comprising: transmitting in a vectortransmission system a sequence of symbols from a first transceiver to asecond transceiver, the sequence of symbols being based on a modulationof a predetermined symbol with an orthogonal sequence of a set oforthogonal sequences; selecting symbols from the sequence of transmittedsymbols for providing far-end crosstalk update information, the selectedsymbols being a subset of the transmitted symbols; providing far-endcrosstalk update information based on a transmission error of arespective selected symbol received at the second transceiver.
 27. Themethod according to claim 26, wherein the sequence of symbols comprisesonly symbols of an orthogonal symbol sequence, the orthogonal symbolsequence being multiple times repeated in the symbol sequence, theorthogonal symbol sequence being based on a modulation of apredetermined symbol with the orthogonal sequence, wherein selectingsymbols from the sequence of transmitted symbols comprises: selecting asymbol; and selecting all other symbols of the orthogonal symbolsequence before selecting said symbol again.
 28. The method according toclaim 26, wherein providing far-end crosstalk update informationincludes providing a slicer error for each of the selected symbols, themethod further comprising updating far-end crosstalk coefficients basedon the slicer error of the selected symbols.
 29. The method according toclaim 26, further comprising: switching from a first operation mode to asecond operation mode, wherein far-end crosstalk update information isprovided based on each symbol of the transmitted sequence of symbols inthe first mode and wherein selecting symbols from the sequence oftransmitted symbols for providing far-end crosstalk update informationis provided in the second mode.
 30. The method according to claim 26,wherein the orthogonal sequence is a Hadamard sequence or a m-sequence.31. The method according to claim 26, wherein the sequence of symbols isa sequence of SYNC-Symbols, each SYNC symbol corresponding to arespective DSL superframe.
 32. The method according to claim 27, whereinselecting symbols from the sequence of transmitted symbols for providingfar-end crosstalk update information comprises: selecting a subsamplingfactor k from a set of numbers containing only odd integer numbers; andselecting every k-th symbol from the sequence of transmitted symbols.33. A device comprising: an input configured to receive a sequence ofsymbols transmitted in a vector transmission system by a firsttransceiver of the vector transmission system, the sequence of symbolsbeing based on a modulation of a predetermined symbol with an orthogonalsequence; a selection circuit configured to select a subset of thetransmitted sequence of symbols; a slicer to generate a slicer error foreach symbol of the selected subset; and a circuit to provide far-endcrosstalk update information by utilizing a slicer error of arespectively selected symbol.
 34. The device according to claim 33,wherein the selection circuit is configured to select the symbols byrepeatedly performing the following: selecting a symbol; discarding apredetermined number of symbols which follow the selected symbol in thesequence; and selecting a further symbol after discarding thepredetermined number of symbols.
 35. The device according to claim 33,further configured to selectively switch from a first mode to a secondmode, the device being further configured to provide the far-endcrosstalk update information in the first mode based on a slicer errorof each symbol of the transmitted sequence of symbols and to provide thefar-end crosstalk update information in the second mode based only onthe slicer error for the symbols contained in the selected subset ofsymbols.
 36. A vector transmission device comprising: a symbol generatorconfigured to generate a sequence of symbols, the sequence of symbolsbased on a modulation of a predetermined symbol with an orthogonalsequence; and a selection circuit configured to provide selectioninformation indicating which symbols of the sequence of symbols are tobe used for updating far-end crosstalk coefficients and which symbols ofthe sequence of symbols are not to be used for updating far-endcrosstalk coefficients.
 37. The vector transmission device according toclaim 36, wherein the selection circuit is configured to select thesymbols by selecting every k-th symbol from the sequence of symbols,wherein k is only an odd integer number.
 38. The vector transmissiondevice according to claim 36, further configured to selectively switchfrom a first mode to a second mode, the device being further configuredto provide far-end crosstalk update information in the first mode basedon a slicer error of each symbol of the transmitted sequence of symbolsand to provide far-end crosstalk update information in the second modebased on a slicer error of only the symbols of the selected subset. 39.A vector transmission system comprising: a first transceiver, the firsttransceiver comprising a symbol generator configured to generate asequence of symbols, the sequence of symbols representing amultiplication of a predetermined symbol with an orthogonal sequence; aselection circuit configured to indicate symbols of the sequence ofsymbols which are selected for a far-end crosstalk compensation update;a second transceiver, the second transceiver comprising: an inputconfigured to receive the sequence of symbols; and a slicer configuredto provide a slicer error for the symbols indicated to be selected forfar-end crosstalk compensation update; the vector transmission systemfurther comprising an update circuit, the update circuit beingconfigured to update far-end crosstalk coefficients based on the slicererror of the selected symbols.
 40. The vector transmission systemaccording to claim 39, wherein the selection circuit is configured toselect symbols from the sequence of symbols by selecting every k-thsymbol from the sequence of symbols, wherein k is a selectable oddinteger number.